I never had an issue with a number of X4s over the years running bars up to 720mm. Then I put 760mm Nukeproofs on to my 90mm Thomson and you could really notice the flex. Changed to an 85mm Haven which is much stiffer. Whether you’d notice much difference when riding is debatable but the flex is there. Running 780 bars with a 70mm Haven now, solid.

No problem with mine on 780mm bars and I am 18 stone honking on them!
Did however find that race face ride 50mm stem and bars are like rubber so swapped thm out for RaceFace Atlas set up at full width, lovely and stiff!